1. Field of the Invention
This invention relates to bottom hole assemblies for drilling oilfield wellbores and, more particularly, to the use of electrolytic tilt sensors for determining the inclination of a wellbore.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
To obtain hydrocarbons such as oil and gas, wellbores (also referred to as boreholes) are drilled into the earth by rotating a drill bit attached at the end of a drilling assembling generally referred to as a “Bottom Hole Assembly” (BHA). In some cases, directional drilling activity may be utilized to produce highly deviated and/or substantially horizontal wellbores. For example, a directional well may be desirable to increase hydrocarbon production and/or to navigate drilling activity towards a remote location. Due to the high cost of directional drilling activity, however, the majority of current drilling activity is focused on producing substantially vertical wellbores. As such, wellbores may be drilled in substantially any direction or directions from the Earth's surface to a “target zone”, the path between which is carefully planned prior to drilling. Due to the cost of drilling and the need for restricting drilling activity to the planned wellbore path, however, it is essential to periodically monitor the position and direction of the BHA during drilling operations.
Due to the high cost of directional drilling, about 70% of wellbores are planned and drilled vertically. These vertical wellbores require a means for demonstrating “verticality” (i.e., demonstrating that the well is being drilled in a substantially vertical plane) through the drilling process. To determine the wellbore verticality or inclination, a well survey may be conducted by periodically lowering or dropping a survey tool, or “well logging instrument”, into the wellbore. For example, a critical vertical drift (CVD) tool is a survey tool commonly used to measure the inclination of vertical wellbores. In general, the CVD tool includes an elongated tubular housing, which is centered within a survey barrel and contains the operating elements of the tool. To conduct a well survey, the survey barrel (otherwise referred to as “running gear”) is dropped or lowered into the wellbore to position the survey barrel in alignment with a longitudinal axis of the wellbore.
More specifically, the survey barrel may be run into the wellbore by a wire line cable, which is spooled from a cable drum mounted on the drill floor of a drilling rig. In this manner, the cable drum functions to raise and lower the survey barrel within a drill string, which is connected at one end to the bottom hole assembly at the drill bit. When drilling is temporarily stopped at a particular wellbore depth, the survey barrel is dropped or lowered into the drill string to land on a centralizing ring arranged above the drill bit. Such a ring is generally referred to as a “landing ring” or “Totco ring.” Consequently, CVD tools are also known as “drift tools” or a “Totco.” The landing ring functions to position the survey barrel in the direction of the BHA, and thus, in rough alignment with the axis of the wellbore. To eliminate some of the shock associated with landing on the ring, a shock absorber or “shock subassembly” may be incorporated at the bottom of the running gear.
The housing portion of the CVD tool generally includes a pendulum having a sharply pointed projection at its lower end. The pendulum is free to pivot in any direction, and thus, is able to maintain a vertical position regardless of the inclination of the housing. As the housing is inclined in accordance with the direction of the wellbore, the axis of the pendulum becomes offset from the axis of the housing by an amount proportional to the angular inclination of the wellbore. Such an “inclination angle” is described herein as the angular deviation between a longitudinal axis of the wellbore and the gravitational vector.
In addition to the pendulum, the housing includes timing and recording elements, which control the sliding movement of a chart holder coupled to one end of the recording element. The chart holder carries a disk-like chart typically constructed of thin metal or paper and includes a plurality of equally spaced concentric circles printed thereon. In most cases, the space between each of the concentric circles indicates one or more degrees of inclination. After a predetermined time, in which the pendulum is allowed to come to rest after landing, the recording elements causes the chart holder to move upwardly, thereby engaging the disk-like chart with the pointed projection of the pendulum and producing a perforation in the disk. Subsequently, the disk may (or may not) be rotated to another angular position before a second engagement between the disk and pendulum produces a second perforation.
After completion of the second engagement, the CVD tool is withdrawn from the wellbore and retrieved at the surface for examination by an operator. The position of the perforations on the disk relative to the concentric circles provide the measured wellbore inclination at the time and wellbore depth the survey was taken. A reading in the center of the disk indicates a substantially vertical inclination measurement, whereas an “off center” reading indicates the amount of wellbore inclination. The first and second perforations may also be compared against one another for determining a general accuracy of the overall inclination measurement.
Conventional CVD tools, however, present several disadvantages when used in current drilling operations. For example, reading of the disk in not only subjective, but also difficult due to the small size of the disk. In most cases, the disk must be read under a magnifying element to obtain a reading. As such, the accuracy of the reading may be compromised due to operator subjectivity. Another disadvantage of conventional CVD tools is the constraint on operation within specific ranges of inclination angles. As such, an approximate wellbore inclination angle must be known prior to conducting a well survey. Otherwise, an inaccurate reading may result when the wellbore inclination angle is outside an operational range of the particular CVD tool used to conduct the well survey.
Furthermore, conventional CVD tools use a mechanical timing element, which is set at the surface for delaying the recordings (i.e., the first and second engagements) by predetermined and estimated amounts of time. As such, conventional CVD tools lack a means for automatically detecting the occurrence of a “landing event” (i.e., the shock detected when a tool lands at the bottom of the wellbore). In addition, conventional CVD tools cannot distinguish between a landing event and other “shock events” (i.e., vibration due to motion of the tool through a wellbore and/or due to wire line cable problems). Therefore, an inaccurate reading may result when the wellbore inclination angle is recorded at the wrong time and/or place within the wellbore.
Yet another disadvantage of conventional CVD tools is the limited number of measurements allowed during a single survey. As noted above, conventional CVD tools obtain a maximum of two readings (i.e., first and second perforations) before the device must be retrieved and the disk replaced. In this manner, conventional CVD tools do not allow a plurality of inclination measurements to be recorded during a single well survey. Since conventional CVD tools record data mechanically, they are not conducive to electronic storage and processing of the measurement data. Finally, the cost of using and servicing conventional CVD tools continues to increase as the technology associated with such tools becomes increasingly outdated.
While many electronic tools have been developed to address the problems outlined above, the basic mechanical CVD tool is still used in most drilling operations today. Reasons for failure of the industry to accept such electronic tools may include, but are not limited to, undesirable in size, cost and ease of use, as compared to the conventional device. Therefore a need exists for a drop-in replacement of the conventional CVD tool. Preferably, such a drop-in replacement would be of substantially equivalent size, cost, and ease of use as compared to conventional CVD tools without suffering from the disadvantages described above.